13. Python - Pandas - Indexing/Selecting/Assigning

 Selecting Columns:

data_frame.column_name // use a method way

data_frame['column_name'] // indexing way


Selecting Data In Columns:

data_frame['column_name'][indexnumber] // indexing way


Selecting with 'iloc' and 'loc' :

    Selecting a Column/Row with iloc:

    data_frame.iloc[rowindex]


    data_frame.iloc[ rowstart_index:rowend_index , column index]

    // index can be negative which means start from the end


    data_frame.iloc[[1,2,3],0]

    // select a list of specified indexes in first column

    // colon species all. if use colon instead of '0' then it selects all columns


    Selecting a Column/Row with loc:

    data_frame.loc[rowstart_index:rowend_index, ['COLUMNA','COLUMNB']]

    // loc uses a list of column to select


    Selecting using Conditions:

    data_frame.loc[data_frame.COLUMN == 'Value']

    // selects only in COLUMN that has that 'Value'

    

    data_frame.loc[(data_frame.COLUMNA == 'Value') | (data_frame.COLUMNB >= 50)]

    // two conditions. Use '&' or '|' 


    data_frame.loc[data_frame.COLUMN.isin(['VALUEA' , 'VALUEB'])]

    // lets use 'isin' instead of conditional symbols. provide a list of values as condition to check if in 

        column and then select those column/rows


    data_frame.loc[data_frame.COLUMN.notnull()]

    data_frame.loc[data_frame.COLUMN.isnull()]

    // using 'notnull()' or 'isnull()' to select null rows


Setting Custom Index:

data_frame.set_index("COLUMNNAME_INDEXTOUSE")


Assigning Values:

data_frame['COLUMNA'] = 'FOOT'

// this is constant assigning and makes every value in COLUMNA equal to 'FOOT'


Iterable Values:

data_frame['COLUMNA'] = range(len(data_frame),0,-1)

// loops from length of data_frame to '0' by decrementing 1 and assigning it as the value each loop

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